Phase and frequency modulation receiver



June 13, 1944. R R 4 2,351,240

PHASE AND FREQUENCY MODULATION REQEIVER Filed NOV. 13, 1941 k f .A.A.A..A.A.A. ,4 INVENTOR q [f BY llllllll 1A ATTORNEY Patented June 13, 1944 '2,351,24o PHASE AND FREQUENC mopvmrron RECEIVE Bertram Trevor, Riverhead, N. Y., assignpeto. Radio Corporation of America, a eorporationot;

Delaware Application November 13, 1941, Serial No. 418,927 12 Claims. (01. 250-320 My present invention relates to receiving systems adapted to receive phase or frequency modulated carrier waves, and more especially to improvements in the discriminator, detector, and audio frequency circuits of ultra-high frequency control and relay receivers employed in phase modulated carrier wave reception.

' There has been disclosed by G. Mountjoy in application Serial No. 345,903, filed July 17, 1940, granted April 21, 1942, as U. S. Patent No. 2,280,- 536, a discriminator-detector circuit for phase or frequency modulated carrier waves. In that detector circuit the signal input circuits of the rectifiers are oppositely and equally mistuned from the center frequency of the applied modulated waves, and symmetry is provided in the detection characteristics by utilizing mutual inductance between the input circuits to cancel the effects of any capacitance coupling thereto from prior networks.

' It is one of the main objects of my present invention to provide in a receiver of angular velocity-modulated carrier waves, a detector of the type disclosed in the aforesaid Mountjoy patcm which is further improved so that a perfectly balanced discriminator is had for all frequencies of modulation.

' Another important object of my present invention is to provide in an ultra-high frequency receiving system for phase modulated carrier waves, a perfectly balanced discriminator-detector network which includes novel modulation voltage coupling networks subsequent to the detector in order to eliminate undesirable effects caused by the presence of amplitude modulation components which may be passed by an imperfect limiter.

Still another object of the invention may be stated to reside in the provision of a relatively wide band phase or frequency modulation re.- ceiver, such as may be employed for facsimile, television or multiplex communication, and wherein there is utilized a discriminator-detector network of the type disclosed in the aforesaid Mountjoy patent, but which detector terminates in a network functioning to produce a flat modu lation voltage response over the entire modulation frequency spectrum.

Still other objects of my invention may be stated to reside 'in the provision of improved discriminator and audio frequency circuits for ultra-high frequency control and relay receivers utilized for: phase modulation reception, and particularly in circuits which are economically and easily manufactured and assembled.

The novel features which I- believe to be characteristic of my invention are set forth in particularity in the appendedclaims the invention itself, h ver. both rg n a on an method of operationwi-ll best be understood by reference to the following description taken in connection with the" drawing in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect,

In t dra in Fig. 1 shows only that portion of a receiving system embodying my present invention, 'F 2 sho a d ifl at o e e n ow gflth cc m a y n draw n wherein like reference characters in the two figures designate similar circuit elements, by way of illustration it is pointed out that the receiv ing system shown in Fig. 1 is embodied in an ultra-high frequency control and relay receiver utilized for phase. modulation reception. only: such of the networks of such a receiving system are shown as is necessary to a proper under! standing of the present invention. The receiving system is of the superheterodyne type, and emp ys a p rati inte me i re cy (I. F.) of, for example. 8*megacycles (me). I. F. band width iswide, and may be of the order of 200 kilocycles (kc).

h n tw rk pr edins helr F- n w k av n t bee sho n. Since ho e kl le 'i h art are. fully acquainted. ith the manner of collecting the r d ted ltrarh sh n y mod ate carrier waves. They are also fully aware of the means that can be used to convert the ultrahigh frequency modulatedwaves to the modulated carrier waves whose center frequency (Fe) is ofthe operating I. F. value. The radiated ultra high frequency waves may have a center free qency of between 100 and 500 me. if desired.

The I; F. energy output of the converter may be transmitted through as many I;. F. amplifier stages as is desired. To preserve simplicity of disclosure, there is shown in Fig. 1 but a single I. F. amplifier tube I. The signal input grid of the latter is connected to the high potential side of the resonant input circuit '2 which is shunted by the damping resistor 3. The circuit 4, shunted by damping resistor 5, is magnetically coupled to the input circuit 2, andfeach of the, circuits 4 and 2 is resonated to the operating I; v lue. Th ent re. n tw rk. 5+ Z'-?. as an ass. har c e i ti h e; pa s band. s 1 kc. wide; so that the modulated carrier waye may be transmitted to the amplifier I with subtdthe' grids of the controlled amplifiers.

2 stantial uniformity. The I. F. amplifier tube I is of the usual and well known construction, and its plate circuit is provided with a resonant output circuit 6 which is tuned to the I. F. value. The circuit 6 is magnetically coupled to the following resonant circuit 1.

Here, again, the network 6-'| is constructed as a band pass network' whose pass band width is 200 kc. wide. The circuit 1 feeds its modulated carrier wave energy into a limiter tube 8 I The limiter may be constructed in any well known manner, and may include the grid condenser 9 and leak resistor lllwhose constants are chosen to provide amplitude modulation limiting action. Those skilled inthe art are fully aware of the manner of 'f-unction'ing' of the limiter, and realize that it acts to eliminate most of the amplitude modulationfifiects existing on the carrier. These effects may arise due to noise impulses, fading or even because of the passage of the waves through thevarious cascaded resonantcircuits up to the limiter tube. The voltagesof'the various cold. electrodes of the limiter tube'8 are chosen so as to render tube 8 'areadilysaturated amplifier whose output amplitude above-a predetermined level is substantially fiat.

"Automatic volume control (AVC) may be provided so as to improve the action at the limiter stage, as .well as to compensate for; carrier fading atthe signal collector. This is easily accomplished in any well known manner and, in Fig. 1 thereis shown one-method of securing 'AVC bias for the control. grids ofv theultrla-high frequency amplifiers or I. F. amplifiers whichiam to be controlled in .gain. A diodel may have its anode connected to the high potential side of the limiter inputcircuit '|.',.whi1e the cathode of diode H is connected through condenser 2 to the low potential sideof input circuit I. i

l The low potential terminal of the coil of input circuit 1 may be connected to a source of negative voltage so that the diode H and associated load .circuit' l4, L3 is maintained at a substantial negativepotential in order to cooperate properly with the direct currentamplifier |5,- which supplies the control potentials tothe controlled tubes. Thenumeral l3 designatesfthe load resistor of the'diode ||,,and numeral l4 designates a filter network which prevents I- F. energy from being fed to the load resistor l3 'or D. C. amplifier I5.

. The voltage tapped off from the cathode 'end of resistor- I3 may be applied to the directcurrent and the amplified AVCbias may then be applied polarity of the direct current voltage at the positive end of resistor |3 so that it is readily applied plifiers." I p v Th ";discriminator detector network comprises apair of resonant circuits F2 and Fi'arranged in series in'the plate circuit of the limiter tube 8. The proper positive voltage is applied to the plate of limiter 8"through the coils l6 and ll of the series tuned circuits. The coils H5 and H are magnetically coupled, as indicated by M. The low, potential end of coil is by-passed to ground by condenser l8. The diode i9 is operatively associatedwith tuned circuit F2, while diode 2B is associated with the tuned circuit Fr. The resistor 2|"func tio'ns as the load resistor fordiode l9 and isconnected in shunt therewith, while resistor 22 is the load,resistor for'diodefn and 'shunts the latter..- 1

voltage amplifier l5 of any'desired construction,'

Of course, theamplifier IE-functiOns to, reverse the input circuits F1 and F2 are oppositely mistuned with respect to the center frequency Fe. That is, tuned circuit F2 will be tuned to one side of the operating 1. P. value, say a frequency spacing of 0.1 me. The input circuit F1 would then be mistuned to the opposite side of F0 by the same 0.1 mc. amount.

As disclosed in the aforesaid Mountjoy patent, there is an inherent circuit. capacity coupled between the prior networks and the detector input circuits. Such. capacity isdue to the plate to ground capacity of the limiter tube plus the attendant plate winding and circuit capacitance to ground. This inherent capacity tends to ren der the detection characteristic unsymmetrical. Proper poling and adjustment of M, the mutual inductance between coils I6 and I1, will cancel out the effect of the undesired inherent capacity, and a symmetrical detection characteristic results. These series circuits are tied directly across the limiter plate circuit. w

Th maximum rectified voltage appears across resistor 2| when the instantaneous frequency of the modulated carrier energy is substantially that of the circuit F2. Similarly, maximum rectified voltage appears across resistor 22 when the modulated carrier energy frequency is equal to the resonant frequency of circuitFi. Between these two frequencies of the frequency modulated carrier the rectified voltage across each. of load resistors 2| and 22 will depend upon the frequency displacement of the instantaneous carrier frequency with respect to the resonant frequencies of each of the input circuits F2 and F1. If the low potential end of resistor 22 were directly grounded, as shown in Fig. 2, the effective output voltage at the upper end of resistor 2| would depend upon the difference between the rectified voltages produced across resistors 2| and 22.

However, in Fig. 1 the input grids of a pushpull audio amplifiervare driven from the opposite ends of load resistors 2| and 22, and, therefore, it will be seen that the detector output load in Fig. 1 is arranged for push-pullv audio operation. Since the audio frequency voltage components are represented by frequency deviations of ,the" carrier, it will be seen that the voltage takenoffat the opposite ends of resistors 2| and 22'is the audio modulation voltage. As is well known to those skilled in the art, in phase modulation a relatively greater increase of frequency deviation of the carrier occurs at the upper audio frequencies compared with the deviation at low audio frequencies. That is, the upper audio frequencies are emphasized by producing a relatively wider de-emphasizes the higher audio frequencies so,

that there is secured a flat audio response over the audio frequency spectrum. In the absence of such a correction network the output of the. detector would rise with frequency when phase,

modulated carrier waves are received.

' The audio. amplifier. tubes. 30 and 30' have their cathodes connected in common, and the. common cathodes are connected to round through a resistor 35. 'Thegrid of -tube 30 islconnected to the anode ofdiode |9 through a path which includes condenser 3|, and resistor. 32 arranged in series, the junction of condenser 3| and resistor 32 being connected to ground through a condenser 33. Similarly the grid of tube 30' is connected to the anode of diode through a path which includes condenser 3| and resistor 32." arranged in series, the. junction of the latter two elements being connected to ground through condenser 33'. Condensers 33 and and 33 .with their associated resistors 32 and 32' provide the correction network. referred to above. Furthermore, the anode of diode I9 is. connected to the anode of diode 20 through a pair of series re-' sistors 34 and .34 whose junction is grounded. Automatic frequency control (AFC) voltages may be taken off from the opposite ends of the load resistors 2| and 22.v

This is accomplished by connecting. the junction of. resistor 32 and condenser 3| to ground through a path which includes resistor 40 and condenser 4| in series. The junction of resistor 4|) and condenser. 4| is connected to one AFC bias lead. Similarly, a second AFC- bias lead is connected to the junctionof. resistor 40' and condenser 4| arranged in series between the junction of resistor 32' and condenser 3 If and ground. The potentials on these AFC- leads control a balanced D. C. amplifier. which in turn controls the frequency of the local oscillator employed in connection with the first, or second, detector as the case may. be. Briefly,the AFC network acts to correct the local oscillator frequency in a sense to maintain the operating I. F. value. A push-pull AFC system of the type'that can be used in the present case is. shown in my U. S. Patent A perfectly balanced discriminator is had for all frequencies of demodulation if the three capacitances 24, 23, 2-5 are made equal, and have high reactances compared with resistors 2|, 22 for all modulation frequencie used. In that case any amplitude modulation components which are passed by the limiter are perfectly balancedv out, and do not appear in the output of the audio frequencynetwork. Perfect balance is obtained only if the values of the capacities of condensers 24, 23,. and 25 are equal as well as the corresponding resistors and condensers (referred to with numerals and corresponding primes) inthe coupling network between the diodes 9 and 20 and the audio amplifier tubes 39 and The audio amplifier tube is supplied from the plate circuit of tube 5|. The plate of tube 5| is connected to the plate of tube 30, the positive plate voltage being supplied to these plates through resistor 52. The plate of tube 39' is connected to the positive voltage source through a resistor. The plate of tube 30' is connected-to ground through aseries path comprising condenser 54, condenser and resistor 56. The condenser 55- is shunted by resistor 51. The adjustable tap 58 is slidable along potentiometer resistor 56. Tap 58 is connected to the control grid of tube 5|. The direct current voltage developed across bias resistor 35, the latter being shunted by a bypass condenser, is used to provide bias for the grids of tubes 30, 3B and 5|.

The grid of'tube 59 is connected to the adjustable tap 60 of potentiometer resistor 6| The latter has. its lower. end grounded, while its upper end is coupled by condenser 62. to the plate of tube 5|. The tube50 has a self-biasing resistor 63 in its cathodecircuit. The. audio voltage developed across the plate load resistor 10 is transmitted to the succeeding audio utilization network. i

In the connection between condenser 3|" and the junction of resistors 32'-40' there is located a switch arm 89 which is adjustable. between contacts P and B. The contact B is connected by lead 8| .to the left terminal of condenser. 3|. Normal angular modulation componentsappear push-pullbetween the grids. of tubes 39 and 30' when the switch arm is on contact P (operation connection). The;balance potentiometer 56-58 is adjusted to feed the grid of tube 5| with exactly the same voltage that appears at the grid of tube 39. Due to the 180 degree phase reversal at tube 30, the voltages on the grids of tubes 39 and 5| arein. phase and are equal in magnitude. Therefore, tubes 30 and 5| operate effectively in parallel to drive the control grid of tube 5|] through the gain potentiometer Gil-5| Should any undesired amplitude modulation components pass through the limiter, a pushpush input is applied to the grids of tubes 30. and 30'. It will now be'seen that the voltages on the control grids of tubes 30 and 5-|- are equal in magnitude but opposite in phase. This is the condition for perfect balance, since no amplitude modulation inputreaches the grid of tube 5|. This elfectively eliminates unwanted amplitude modulation components ifthe discriminator is well balanced. in the. manner described above. The purpose of switch 89 is to permit an accurate adjustment of the balance-- potentiometer 58. This adjustment is made by throwing the switch in the B position which ties. the grids, of tubes 30 and 39' in parallel. The potentiometer 58 is then adjusted for. zero'outputof tube 5|lwith normal input to tubes 30-30. In Fig. 2 there is shown the manner in which the demodulator output load is arranged for feeding a single audio channel as disclosed in the said Mountjoy patent. The correction network comprises resistor 99 arranged in series with con denser 9|, and the series. path is connected in shunt across the two series-arranged diodes l9 and 29. The AFC bias. may be taken off from the junction of resistor 90 and condenser 9|. The resistor-condenser filter. 92'functions to filter out any pulsation voltage components in the AFC bias. The audio voltage is taken off from the junction of resistor 90 and condenser 9|, and is transmitted through condenser 91 to the control grid of audio amplifier 93,. A proper bias may be applied to the control grid 93 through a resistor 94, and the cathode. of tube 93 is connected to ground through. a load resistor 95.. Audio voltage for, the following audio frequency network may. be taken off. in any desired magnitude by means of the adjustable tap 96 which is slidable along resistor 95. l

The advantage of the present invention is best appreciated by comparison of the arrangement of Fig. 1 with'that shown in Fig. 2..

' First, it should be pointed out that the single ended arrangement shown in Fig. 2 does not provide a substantially perfect balance against AM components passing an imperfect limiter at the higher audio frequencieswhere appreciable audio frequency current flows through the capacitors 23 and 24. In this circuit amplitude modulation components: passing the limiter pro duce equal and opposite voltages across. resistors 2| and 22, assuming these-two resistors have equal values of resistance. Then if no appreciable audio frequency current flows through'the capacitors 23 and-Z4,- we have a balanced condition which gives no audio output for the AM age divider 34, 34- which grounds the midpoint of the discriminator outputfor both direct current. and alternating. current potentials. The capacitors 3| and 31' are merely couplingcapacitors to the grids of tubes and 30' respectively. it, now, we have AM components passing the limiter, we again haveequal and opposite voltages developed across theresistors 2| and 22.":If the three'capa'citors 23, 24, 25 are equal in reactan'ce, any audio frequency current flowing in them does not disturb the balance, but we do get a push-push input'to the audio frequency amplifier which is so" balanced that it will not respond to this type of input.

While I have indicated and described several systems for carrying my invention into efiect, it will be apparent to-one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many 'modifications'may be made without departing from the scope of my invention, as set forth in the appended claims.

WhatIclaim is:

1. In a receiver of phase modulated carrier waves; a" limiter tube provided with input and output electrodes, a discriminator-detector network following the limiter, said network comprising-a diode rectifier circuit provided with a resonant input circuit and a resistive output load, a second diode rectifier circuit provided with a second resonant input circuit and a second resistive output load, means connecting said resistive loads in phase opposition, said input circuits being oppositely and equally mistuned relatively to the center frequency of said waves and arranged in series relation with each other and in series with the space discharge paths of said rectifier diodes, said resistive loads being connected inseries between the anodes of said diodes,

said loads being connected directly in shunt with said series arranged input circuits, a condenser connecting the junction of the resistive loads to the junction of said input circuits, 9. second condenser connecting the anode end of one resistive load to that side of the input circuit associated therewith, a third condenser between the anode end of the secondiresistive load and that side .oi the input circuit associated therewith, said three condensers being of equal value, a resistive path in shunt with said diodes, the midpoint of said last path being grounded, and a balanced push-pull audio amplifier coupled to said resistive path thereby to provide a perfect balance for amplitude modulation components. 2.-In-- a receiver of phase modulated carrier waves, a limiter tube provided with input and output. electrodes, a discriminator-detector network following the limiter, said network comprising a diode rectifier circuit provided with a resonant input circuit and a resistive output load,

a second diode rectifiercircuit provided with a second resonant input circuit and a second resistive output load, means connecting said'resistive loads in phase opposition, said input circuits being oppositely and equally mistuned relatively to the center frequency of said waves and arranged in series relation with each other and in series with the space discharge paths of said rectifier diodes, said resistive loads being connected in series between theanodes of said diodes, said load being connected directly in shunt with said series arranged input circuits, a condenser connecting the junction of the resistive loads to the junction of said input circuits, at second condenser connecting the anode end of one resistive load to-that side of the input circuit associated therewith, a third condenser between the anode end of the second resistive load and that side of the input circuit associated therewith, said three condensers being'of equal value, a separate-correction network connected in shunt across each of said diodes to provide a flat audio response over the audio frequency spectrum, and a balanced push-pull audio amplifier, coupled to said correction networks, which is unresponsive to push-pushinput.voltages.

3. In combination with a demodulator for angular-velocity modulated carrier waves of the type wherein a plurality of different types of modulation exist on a common carrier, a demodulator comprising-a :pair of" opposed rectifiers having separate input circuit and load'resistors, said input circuits 'beingequally and oppositely mistuned relative to the center frequency of the modulatedcar-rier waves, a push-pull amplifier having its'input electrodes connected to the opposed diodes to derive modulation voltage from said resistive loads, an inverter stage to convert the push-pull amplifier to a singleended one, said inverter stage including an electron discharge tube whose plate is connected in parallel with the plate of one of the push-pull amplifier tubes and whose input grid is crossconnected to the plate of the opposite one of said push-pull amplifier tubes and a balancing potentiometer connected: between the inverter grid and the last named plate.

4. In a phasemodulation receiver, a limited tube provided with input and output electrodes, a discriminator-detector network following the limiter, said network comprising a diode rectifier circuit provided with a resonant input circuit and a resistive output load, a second diode rectifier circuit provided with a second resonant input circuit and a second resistive output load,

means connecting said resistive loads in phase.

opposition, said input circuits being in seriesrelation and oppositely and equally mistuned relatively to the center frequencyof said waves, a condenser connecting the junction of th re- .sistive loads to the junction of said input circuits, a second condenser connecting the anode end of one resistive load to that side of the input circuit associated therewith, a third condenser between the anode end of the second resistive load and that side of the input circuit associated therewith, said three condensers being of equal value, a pair of symmetrical correction circuits connected to said diodes, and a pair of series-arranged balancing resistors connected across said diodes.

5. In combination, a limiter tube provided with input and output electrodes, a detector network following the l imiter,-said network comprising a diode rectifier circuit provided with a resonant nput, ci it a d a resis ve o tput oad, a se hd diode r c e ci cuit ovided wi a econd sonant i u c r u t and ec nd res s iv utput load mean co ect n said; res s ve oad n p ase oppos n a d i pu circ i be n 53f fies-relate a d o os e and equa y. mist ne elat ve to the e ter re u c of sa wav a con ns r Qnh ct e the ju ct o o h s s ive. a s o e i n n of a d. i ut cirits a second conde ser connectin he nod end of one resistive load to that side of the input circuit associated therewith, a third condenser between the, anode end of the second resistive load and that side of the input circuit associated therewith, a balanced push-pull amplifier, unresponsive to push-push input voltages, connected to said diodes, and a phase inverter connected to said balanced amplifier to provide a single-ended amplifier circuit.

6. In combination with a demodulator for angular-velocity modulated carrier waves of the type wherein a plurality of different types of modulation exist on a common carrier, a demodulator comprising a pair of opposed rectifiers having separate input circuits and load resistors, said input circuits being equally and oppositely mistuned relative to the center frequency of the modulated carrier waves, a balanced push-pull amplifier having its input electrodes connected to the opposed diodes to derive angular modulation voltage from said resistive loads, an inverter stage coupling an angular modulation channel to the output of said push-pull amplifier, said inverter stage including a tube whose plate is connected to the plate of a given one of the pushpull amplifier tubes and whose input grid is crossconnected to the plate of the opposite one of said push-pull amplifier tubes, means for selectively connecting the grids of the push-pull amplifier in common, and a balance potentiometer connecting the inverter input grid to the last named plate.

'7. In combination with a demodulator for phase modulated carrier waves of the type wherein a demodulator comprises a pair of opposed rectifiers having separate input circuits and load resistors, said input circuits being equally and oppositely mistuned relative to the center frequency of the modulated carrier waves, a push-pull amplifier having its input electrodes connected to the opposed diodes to derive the phase modulation voltage from said resistive loads, means for selectively connecting the push-pull amplifier input electrodes to a common point of said load resistors, a phase modulation voltage utilization channel, an inverter stage coupling said channel to the output of said push-pull amplifier, and a pair of symmetrical correction circuits connected between said diodes and the push-pull amplifier input electrodes.

8. In combination with a discriminator-detector network for phase modulated carrier waves comprising a diode rectifier circuit provided with a resonant input circuit and a resistive output load, a second diode rectifier circuit provided with a second resonant input circuit and a second resistive output load, means connecting said resistive loads in phase opposition, said input circuits being oppositely and equally mistuned relatively to the center frequency of said waves and arranged in series relation with each other and in series with the space discharge paths of said rectifier diodes, said resistive loads being connected in series between the anodes of said diodes, said loads being connected directly in shunt i h sa sc e a ran ed np t c rcu s the provement comprising a condenser connecting.

shunt w h sa d d od s a d t e. d i of sa s ath bei o ded- 9, In a dis rimi atoretect ne w rk phase modulated carrier waves, a diode rectifier circuit provided with a resonant input circuit and a resistive output load, a second diode rectifier circuit provided with a second resonant input circuit and a second resistive output load, means connecting said resistive loads in phase opposition, said input circuits being oppositely and equally mistuned relatively to the center frequency of said waves and arranged in series relation with each other and in series with the space discharge paths of said rectifier diodes, said resistive loads being connected in series between the anodes of said diodes, said loads being connected directly in shunt with said series arranged input circuits, a condenser connecting the junction of the resistive loads to the junction of said input circuits, a second condenser connecting the anode end of one resistive load to that side of the input circuit associated therewith, a third condenser between the anode end of the second resistive load and that side of the input circuit associated therewith, said three condensers being of substantially equal value, and a separate correction network connected in shunt across each of said diodes to provide a fiat audio response over the-audi frequency spectrum.

10. In combination with a demodulator for angular-velocity modulated carrier waves of the type wherein a plurality of different types of modulation exist on a common carrier, a demodulator comprising a pair of opposed rectifiers having separate input circuits and load resistors, said input circuits being equally and oppositely mistuned relative to the center frequency of the modulated carrier waves, a push-pull amplifier having its input electrodes connected to the opposed diodes to derive modulation voltage from said resistive loads, means for selectively connecting said input electrodes to a common point of said load resistors, an inverter stage to convert the push-pull amplifier to a single-ended one, said inverter stage including an electron discharge tube whose plate is connected in parallel with the plate of one of the push-pull amplifier tubes and whose input grid is adjustably cross-connected to the plate of the opposite one of said push-pull amplifier tubes.

11. In a discriminator-detector network for angular velocity modulated carrier waves, a diode rectifier circuit provided with a resonant input circuit and a resistive output load, a second diode rectifier circuit provided with a second resonant input circuit and a second resistive output load, means connecting said resistive loads in phase opposition, said input circuits being in series-relation and oppositely and equally mistuned relatively to the center frequency of said waves, a condenser connecting the junction of the resistive loads to the junction of said input circuits, a second condenser connecting the anode end of one resistive load to that side of the input circuit associated therewith, a third concenter frequency of applied frequency-variable waves, a condenser connecting the junction of the resistive loads to the'junction of said input circuits, a second condenser connecting the anode end of one resistive load to that side of the input circuit associated therewith, a third condenser between the anode end of the second resistive load and that side of the input circuit associated therewith, and a balanced push-pull amplifier, unresponsive to push-push input voltages, connected to said diodes.

BERTRAM TREVOR. 

